植被覆盖对沙尘天气滞后性影响的机制分析

1982～2000年卫星和335个气象台站沙尘天气观测资料相关分析显示,中国区域沙尘天气年发生频次与植被覆盖率存在非线性关系.在沙尘天气主要发生源地，夏季植被的覆盖状况会直接影响后期冬春两季各类沙尘天气的发生次数,而这种影响机制却不清楚.本研究通过准地转正压模式，从3个摩擦量级分析了上述现象的物理机制.结果表明，在大气与水面摩擦系数量级，最高可在72 h内使边界层大气风速平均下降90%；在大气与裸土摩擦量级，18 h内边界层大气平均风速最高可下降100%；在大气与植被摩擦量级，1 h内边界层大气平均风速就可以下降100%.观测事实和模拟结果证实，地表植被残存的根茎，是影响冬春两季沙尘天气发生频次的重要因素之一.残留根茎对大气的摩擦阻挡作用，是夏季植被对沙尘天气产生滞后性作用的主要机制.

Process Study on Hysteresis of Vegetation Cover Influencing Sand-dust Events

Data analysis from satellite and weather stations during 1982-2000 shows nonlinear relationship between vegetation cover and sand-dust events is present in most part of China. Vegetation cover ratio in summer can impact significantly on the frequency of sand-dust storms from winter to spring in the source regions of sand-dust events. It is not quite clear about the hysteresis that vegetation cover in summer influence sand-dust events during winter and spring. A quasi-geostrophic barotropic model is used under the condition of 3 magnitude of frictional coefficient to investigate the cause of the hysteresis. Wind velocity shows a greatest decline at 90% during 72 h as initial wind velocity is 10 m/s for magnitude of frictional coefficient between atmosphere and water surface, greatest decline at 100% during 18 h for magnitude of frictional coefficient between atmosphere and bare soil and a 100% reduction of wind speed during 1 h for magnitude of frictional coefficient between atmosphere and vegetation cover. Observation and simulation prove that residual root and stem from summer vegetation are one of factors to influence sand-dust events happened during winter and spring. Air inhibition from residual root and stem is a most important reason for hysteresis that vegetation cover influence sand-dust events.